The invention relates to a method for joining at least two adjoining work pieces by friction welding wherein the work pieces include an adjoining area, in which they are plasticized by a rotating pin-like projecting tool which is in contact with the adjacent surface areas of the work pieces and to an apparatus for performing such a method.
A similar method and apparatus are known from EP-B-O 615 480. The friction welding, which is also called friction stir welding (FSW), has been basically known for several years. Originally, the friction stir welding was performed in such a way that two work pieces, which were to be joined by friction welding, were moved relative to each other in the area in which they were to be joined while they were pressed against each other with a predetermined adjustable force. As a result of the heat generated by the friction, the material of the work pieces was plasticized in the area of the jointure. Upon sufficient plastification, the materials of the two workpieces intermix sufficiently so that, upon cooling, the desired weld connection between the two workpieces is formed.
In the method and the apparatus mentioned above, a pin-like projection, which is rotated by a drive or motor at a high speed, is disposed in the adjoining area between two workpieces to be joined and the pin-like projection is guided along the adjoining area for example by a special guide arrangement or by a robot. Additionally, it may be subjected to a translatory movement along the adjoining edges of the workpieces. When, after the beginning of the welding procedure, the material of the workpieces is sufficiently plasticized by the frictional heat generated by the rotation of the pin-like projection, the adjoining workpieces while the rotation of the pin-like projection, the pin-like projection is moved along the seam between the adjoining workpieces while the rotation of the pin-like projection is maintained whereby an elongated seam is formed.
It is known that two or more metallic workpieces can be joined by so-called spot welding connections which, with regard to rivet or screw connections, have the advantage that no joining elements (rivets, screws, etc.) are needed and that the joints can be established very rapidly since no holes have to be drilled through the workpieces for receiving the rivets or screws and that further no rivets or screws have to be inserted through the holes and to be compressed or tightened.
While the spot welding of two workpieces of iron or steel or their alloys to be joined can be performed with commercially available spot welding apparatus, the spot welding of two or more adjacent workpieces of aluminum or aluminum alloys as they are used in airplane or motor vehicle assembly procedures is still problematic. The reason is that aluminum and aluminum alloys form with the oxygen of the ambient air on their surfaces an oxide layer which provides for a high electrical resistance between the workpieces to be joined so that, furthermore, the molten material lens formed between the workpieces includes aluminum oxide which detrimentally affects the strength of the joints. As a result, the spot-welding joints of aluminum workpieces do not have the strength required for many applications.
Particularly in the construction of airplanes where highest qualities of the jointure between workpieces are of utmost importance, the use of spot-welding instead of rivet connections has been abandoned. The same is true in the construction of motor vehicles where, increasingly, aluminum is used for chassis and body components.
It is therefore the object of the present invention to provide a method and apparatus for providing high-strength connections between metallic and also non-metallic workpieces, which do not require additional connecting elements and which are reproducible with high precision and in a simple manner. With the method and apparatus according to the invention oxide-free connections should be established and flat continuous surface areas should be established during the welding at both sides of the jointure area. At the same time, the method and apparatus should be simple and the method should be executable by manufacturing robots.
In a method and apparatus for joining at least two adjoining workpieces by friction welding, a rotatable shaft is provided at its front end with a friction welding tool which is biased toward the workpieces to plasticize the engaged area of the workpiece into which the tool enters while the plasticized material is displaced and stored in, or around, the tool. Upon passing through the top workpiece and entering the lowermost work piece, the tool is removed and the stored plasticized material is returned to the void provided by the tool upon removal thereof from the workpieces for joining the workpieces.
The plasticized material is retained and, upon removal of the pin-like projection, is pressed back into the space voided by the removal of the tool.
The advantage of the method according to the invention resides mainly in the fact that, in contrast to the various known methods for joining two or more workpieces, it is not necessary to subject the areas to be joined to specific pre-treatments. It is not necessary to drill holes into the workpieces, no additional connecting elements are needed, no additional operating step such as the formation of rivet heads or the screwing of screws or bolts or the mounting of nuts, is needed. There is no headxe2x80x94contrary to the bolt friction welding, and no beads of plasticized material or holes or depressions are formed. Furthermore, with the use of workpieces of aluminum or aluminum alloys or other lightweight materials or alloys, only minor amounts of oxides will be included in the joint since the jointure area of all the workpieces being joined are plasticized and the materials are intermixed. No surface area oxide concentrations will therefore be present anywhere in the material joints. Since, with the method according to the invention all the plasticized material is pressed back into the joint area, the surface on the active side, that is, the side of the workpieces from which the pin-like projection extends into the workpieces, becomes again planar while the opposite side remains planar in any case since the pin-like projection does not extend through this side.
In order to prevent that, as a result of the insertion of the rotating pin-like projection into the workpieces that is into the lower workpiece and the displaced material flows to the surrounding area in an uncontrolled manner so that it is not available for forming the jointure, the displaced material is retained in the immediate area around the rotating pin-like projection.
Preferably, the displaced material is returned, upon removal of the pin-like projection, into the void formed thereby at the same rate at which the pin-like projection is retracted from the workpiece, whereby it is made sure that all of the plasticized material is returned and the surface of the workpiece from which the pin-like projection extends into the workpiece is again completely planar after completion of the jointure.
In this way, in contrast to the bolt friction welding, a large force transmitting area is generated which is capable of accommodating also high shear loads.
In order to provide for a rapid procedure, it is advantageous if the workpieces are preheated, in the areas in which the jointure is to be established, from the side from which the procedure is performed and also from the opposite side. This has the advantage that not the whole heat input has to be provided by friction heat. With a controlled preheating only a differential amount of heat in excess of the external heat input has to be provided by friction for the plastification of the material by the friction welding procedure.
In this way, the process according to the invention can be substantially accelerated.
With the apparatus for the jointure of at least two adjacent workpieces in accordance with the method of the invention, the pin-like projection is moved axially through the material of the first workpiece as it is being plasticized at least partially into the second workpiece.
During this procedure, the sleeve surrounding the pin-like projection moves back to permit the pin-like projection to enter the workpiece while still surrounding the welding area.
The advantages achieved by the device according to the invention have been pointed out earlier. The rotational speed of the pin-like projection can be selected depending on the type of workpiece, the number of workpieces and the material of which the workpieces consist. Furthermore, the speed of the pin-like projection may be changed during the establishment of the jointure particularly when the pin-like projection enters a lowermost workpiece, which is also plasticized, in order to prevent a melt-through through the lower surface of the lower workpiece.
In a particular embodiment of the device for joining at least two workpieces disposed on top of one another by friction welding, the workpieces encompass between them a joint area and wherein a rotatable shaft is provided which shaft can be driven at high speed and includes a friction welding tool, the friction welding tool is a sleeve element which is rotated by the shaft. During rotation, the friction welding tool is brought into contact with the connecting area of the workpieces. The friction-welding tool is then moved axially through the material being plasticized until it reaches and enters partially a last lowermost workpiece.
Also for this additional embodiment reference is made to the advantages described earlier. The additional embodiment is advantageous for certain applications if the sleeve element has a greater diameter for producing a wider weld area than the pin-like projection.
Expediently, the pin-like projection is surrounded by a first rotating sleeve element. During axial movement of the pin-like projection into the workpieces or through the workpieces the first sleeve element moves axially in the opposite direction. When the desired penetration depth of the pin-like projection into the workpiece has been achieved and the pin-like projection is retracted from the workpiece, the sleeve element moves toward the workpiece in order to push the plasticized material back into the workpiece.
In connection with the second embodiment, it is advantageous if the sleeve element includes in its interior, a pin which moves in a direction opposite to the axial movement of the sleeve element. When the sleeve element enters the workpiece, the pin within the sleeve element provided for the space accommodating the material being plasticized which is advantageous when larger diameter friction welding areas are formed.
The amount of axial movements of the pin-like projection and of the first sleeve element are so selected that the plasticized material volume displaced by the insertion of the friction welding element can be accommodated and the plasticized material is held under a certain hydrostatic pressure during the whole joining procedure, in order to ensure an optimal jointure to the materials of the interconnected workpieces. In this way, it is made sure in a simple manner that, after completion of the jointure, all of the plasticized material is returned to the plasticized area of the two or of all of the workpieces.
In order to ensure that, during insertion of the pin-like projection into the workpieces as well as during the removal of the pin-like projection from the workpieces, the plasticized material does not flow in an uncontrolled way out of the friction welding area, it is advantageous if the pin-like projection is surrounded by a non-rotating outer second sleeve element by way of which the workpieces are also pressed together at the beginning of the welding procedure. This second outer sleeve element therefore compresses the workpieces in the area of the jointure to be formed with the aid of a counter support structure so that, in the connecting area, there are no gaps between the work pieces. It furthermore forms an outer wall, which prevents the plasticized material displaced by the insertion of the pin-like projection to flow from the friction welding area.
Expediently, in this case, the first sleeve element which rotates together with the pin-like projection is guided in the outer second sleeve element.
In principle, it is possible to provide for the axial movability of the pin-like projection and the axial movability of the first sleeve element in the opposite direction by outside means such as a suitable tool head of a manufacturing robot. However, in order to keep the control sequences of a robot or another manufacturing apparatus which operates the device according to the invention as far as possible away from the manufacturing robot, or respectively, apparatus and in order to be able to rapidly exchange the device in case of malfunctions without the need for reprogramming the manufacturing robot or apparatus the control of the axial movements of the pin-like projection and of the first sleeve element in the opposite direction is preferably provided by a spiral groove guide structure in a control ring element in which control pins are received. In this way, the axial movability of the pin-like projection and the axial movement of the first sleeve element coupled therewith are realized within the device.
To this end, expediently, a housing is provided in which the control ring element is supported so as to be rotatable by means of an operating lever operable from without the housing.
In principle, the pin-like projection may be disposed at the axial end of a drive shaft or it may be part of a drive shaft, which may be suitably mounted to a robot. Since the pin-like projection is subjected to a high mechanical and thermal load, it is advantageous if the pin-like projection is a separate component, which can be easily exchanged whenever this becomes necessary. To this end, a rotatable pin support element is preferably provided in the device on which the pin-like projection is firmly mounted for rotation therewith.
Since, for providing a jointure according to the invention of several workpieces, the pin-like projection needs to move axially toward the workpieces the device would have to be moved in accordance with such axial movement if it would be mounted to a robot or other suitable apparatus. Such movement would have to be provided for example by the operating head of a manufacturing robot. In order to eliminate the need for the robot to perform such movement sequences in both axial directions and to transfer this function to the device itself it would be expedient to provide a shaft which engages the pin support element for rotation with the shaft but is axially movable relative thereto.
Preferably, the pin-like projection is circular in cross-section that is, it is an essentially cylindrical member. However, it is not absolutely necessary that the pin-like projection has a circular cross-section; basically it may have any suitable cross-sectional shape. Also, the sleeve element has preferably a circular cross-section and has an axial opening in which the pin-like projection is received and guided. Also in this case, the pin-like projection does not necessarily have a circular cross-section. It is sufficient if the pin-like projection has a cross-section of such a shape that it can be accommodated within the sleeve element and guided therein in a rotation-symmetrical manner.
Finally, it may also be advantageous to provide a sleeve element with a cross-section for receiving and guiding the pin-like projection with an axial opening which cross-section is not circular. In that case, the cross-section of the pin-like projection is adapted in shape to the cross-section of the opening of the sleeve element, wherein the cross-section of the sleeve element and of the pin-like projection may have any suitable shape.
The invention will be described below in greater detail on the basis of the accompanying schematic drawings.